Effect of salinity, source, and biogeochemical factors on organic nitrogen dynamics between dissolved and particulate pools in surface waters of the Mississippi River plume

A microcosm study was conducted that investigated amino acids dynamics in the Mississippi River Plume (MRP) and subsequently compared to spatial and temporal variations in amino acid concentrations in surface waters of MRP collected during 3 survey cruises (March 2002, October 2002, and April 2004). Particulate and dissolved carbon, nitrogen, and amino acids were analyzed in a microcosm experiment where particulate material was mixed with 2.0, 20.4 and 28.9 salinity treatments. Dissolved amino acids (DAA) in the productive mid-salinity region of the plume had higher binding efficiencies than amino acids from non-productive waters, indicative of the relative importance of phytoplankton sources of reactive amino acids. Basic amino acids adsorbed more readily to particles, while acidic amino acids remained in the dissolved pool, indicating that electrostatic mechanisms affected amino acids partitioning. As riverine particles enter the higher salinity shelf waters of the MRP, changing ionic strength was key in controlling amino acid sorption/desorption kinetics.;Field studies showed that seasonal changes in river discharge were key in determining the primary particulate amino acid (PAA) and DAA concentrations in the MRP. The positive correlation between DAA acidic amino acids and salinity, and negative correlation of DAA basic amino acids and salinity in the MRP was largely controlled by differential partitioning of amino acids. While changing ionic strength in the MRP in part controlled amino abundance and distribution of amino acids, bacterial cycling was likely the more important in controlling factor on the larger pool of PAA and DAA. Amino acids represented the highest %C of POC and DOC at mid-salinities, because this is the zone with typically the highest primary production. Non-protein forming amino acids were significantly higher during the October cruise relative to the spring cruises due to resusupension of shelf sediments due to the passage of Hurricanes Isadore and Lili. I propose that the short-term partitioning of amino acids between dissolved and particulate amino acid pools across salinity gradients in the plume were affected by: (1) electrostatic characteristics of suspended riverine sediment surfaces; (2) the availability of "freshly" produced amino acids from phytoplankton; and (3) functional groups of amino acids.